Oxygen molecular orbital approach

When the bonding is considered in the molecular orbital approach, it can be seen that each oxygen atom contributes one p orbital to yield three molecular orbitals. The orbital overlap combinations lead to the wave functions... 

In the manganate ion there is one unpaired electron, but its behavior cannot be predicted by a ligand-field treatment because the overlap of metal and oxygen orbitals is too great. Instead, a molecular-orbital approach must be used. This is, of course, in no way surprising, for we surely could not expect to have an Mn6+ ion surrounded by Oz ions without a great deal of electron density being drawn from the oxide ions into the Mn orbitals. 

Think About It Often the bond order determined from a molecular orbital diagram corresponds to the number of bonds in the Lewis structure of the molecule. In the case of NO, though, the Lewis structure contains a double bond whereas the molecular orbital approach gives a bond order of 2.5. In fact, the molecular orbital approach gives a bond order that is more consistent with experimental data. The experimental bond enthalpy in NO is 631 kJ/mol, stronger than the tabulated value for a nitrogen-oxygen double bond [M Section 8.9]. 

The n molecular orbitals described so far involve two atoms, so the orbital pictures look the same for the localized bonding model applied to ethylene and the MO approach applied to molecular oxygen. In the organic molecules described in the introduction to this chapter, however, orbitals spread over three or more atoms. Such delocalized n orbitals can form when more than two p orbitals overlap in the appropriate geometry. In this section, we develop a molecular orbital description for three-atom n systems. In the following sections, we apply the results to larger molecules. 

The molecular orbital calculation approaches Klamt (1993,1996) and Medven et al. (1996) developed appear to hold promise as a more "scientific" approach based on sound chemical principles. However, these approaches also require a database for development and presently appear to apply to hydrocarbons (i.e., alkanes, alkenes, and aromatic hydrocarbons) and to fairly simple oxygenated compounds. Hopefully, these molecular orbital cal-culational method will be further extended and explored in the near future. 

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